The theater does make a lot more sense for Inside Out than a ride. Worst-case would be just a "book report" version of the film, like the Fantasyland dark rides. Mostly-static AA's standing around in the movie's most famous scenes. A nice-enough experience the first time, but probably not anything you can make an real emotional connection with (which, considering the themes of the film, would actually be kind of a betrayal...).
It would be popular when it first opened, but I bet it would be a walk-on after a few months.
Hopefully, if they go the ride route, they come up with something original to do with the characters. I could see them sticking to the "tour" concept of JIYI, with Joy taking you on a tour of the mind (but it's a Disney ride, so of course Something Goes Wrong!). Still pretty rote, but at least it could free them a bit from just trying to recreate what we've already seen.
I can't remember, does the current Journey Into Imagination have little screens in the ride cars? Because those could be used to show what the "person" whose head you're occupying is doing (and is reacting to the shenanigans taking place during your ride), plus give you an extra little bonus to focus on if you ride again...